1. Field of the Invention
The present invention relates generally to a double valve. More particularly, the present invention relates to a double valve that allows an unpressurized port and a balance port to be in fluid communication when a pressurized port is closed.
2. Background of the Technology
In most offshore-drilling operations, a wellhead at the sea floor is positioned at the upper end of the subterranean wellbore lined with casing, a blowout-preventer (BOP) stack is mounted to the wellhead, and a lower-marine-riser package (LMRP) is mounted to the BOP stack. The upper end of the LMRP typically includes a flex joint coupled to the lower end of a drilling riser that extends upward to a drilling vessel at the sea surface. A drill string is hung from the drilling vessel through the drilling riser, the LMRP, the BOP stack, and the wellhead into the wellbore.
During drilling operations, drilling fluid, or mud, is pumped from the sea surface down the drill string, and returns up the annulus around the drill string. In the event of a rapid invasion of formation fluid into the annulus, commonly known as a kick, the BOP stack may actuate to seal the annulus and control the fluid pressure in the wellbore. In particular, the BOP stack typically includes a plurality of stacked sets of opposed rams (e.g., pipe rams, shear rams, blind rams, etc.) designed to seal in the wellbore and prevent the release of high-pressure formation fluids from the wellbore and so the BOP stack and LMRP function as pressure control devices. The opposed rams are disposed in cavities that intersect the main bore of the BOP stack and support the rams as they move radially into and out of the main bore. Each set of rams is actuated and transitioned between an open position and a closed position by a pair of actuators. In the open positions, the rams are radially withdrawn from the main bore and do not interfere with hardware that may extend through the main bore. However, in the closed positions, the rams are radially advanced into the main bore to close off and seal the wellbore.
Each ram actuator hydraulically moves a piston within a cylinder to move a drive rod coupled to one of the rams. In particular, pressurized hydraulic fluid is supplied to a first chamber within the cylinder on one side of the piston to move the piston in a first direction and close the corresponding ram; and pressurized hydraulic fluid is supplied to a second chamber within the cylinder on the opposite side of the piston to move the piston in the opposite direction and open the corresponding ram. For relatively deepwater subsea BOP stacks, supplying a sufficient volume and pressure of hydraulic fluid from the surface to actuate a ram may be challenging. Consequently, in many cases, subsea-hydraulic accumulators are employed to supply pressurized hydraulic fluid to the ram actuators. The accumulators may be charged with a finite volume of pressurized fluid at the surface prior to being deployed subsea or after being deployed subsea. The pressure of the charged fluid in the accumulators required to actuate the rams depends on a variety of factors—such as the depth of the BOP stack—and must be carefully controlled to ensure proper operation of the rams. Since subsea accumulators provide a finite and limited volume of pressurized hydraulic fluid (between charges), it is generally desirable to conserve pressurized hydraulic fluid volume within subsea accumulators.
Typically, the force required to close a ram is substantially greater than the force to open the ram. However, with a simple piston-cylinder assembly, the same volume of pressurized hydraulic fluid is required to open as well as close the ram. Thus, opening the ram consumes more pressurized hydraulic fluid volume than is necessary and wastes the finite volume of pressurized hydraulic fluid in the subsea accumulators.
Accordingly, there remains a need in the art for devices, systems, and methods for actuating one or more rams of a subsea BOP stack. Such devices, systems, and methods would be particularly well-received if they offered the potential to reduce the volume of pressurized hydraulic fluid necessary to open the rams.